A main shaft of an automatic work supply device or of an automatic tool replacing device in an NC machine tool is required to be capable of making a rotational movement with high accuracy while freely advancing or retreating in the axial direction. In this connection, a combination of a spline shaft and a spline nut is used as means for supporting an axial movement of the main shaft and transmitting rotational torque to the main shaft.
In particular, in a ball spline, in which a spline nut is assembled to a spline shaft through the intermediation of a large number of balls, preload is imparted to the balls, whereby, when transmission of the rotational torque is effected between the spline nut and the spline shaft, no backlash is generated between the two components, making it possible to cause the spline shaft to advance and retreat smoothly while transmitting the rotational torque thereto.
A known conventional example of a compound movement device using such the ball spline is composed of a spline shaft having a plurality of ball rolling grooves extending in the axial direction, a spline nut assembled to this spline shaft through the intermediation of a large number of balls and capable of reciprocating in the axial direction, a substantially cylindrical housing which accommodates the spline nut and through which the spline shaft passes, and a rotation transmitting member, such as a pulley or a gear, mounted to the housing.
In providing the rotation transmitting member on the housing, if the rotation transmitting member is superimposed on the spline nut, the outer diameter of the rotation transmitting member becomes rather large, resulting in an increase in the device size and an increase in cost. In view of this, there is employed, as the housing, a stepped cylindrical member in which a large diameter portion and a small diameter portion are continuous with each other, with the large diameter portion being used as an accommodating portion for the spline nut, and the small diameter portion being used as a mounting portion for the rotation transmitting member.
On the other hand, when such the stepped cylindrical member is used as the housing and the spline nut and the rotation transmitting member are deviated from each other in the axial direction of the housing, a radial load applied to the housing from the rotation transmitting member (i.e., load perpendicular to the axial direction of the spline shaft) is allowed to act on the spline nut as a moment load, and if there is no support at all for the forward end of the small diameter portion of the housing with respect to the spline shaft, there is a disadvantage in that the housing will incline with respect to the spline shaft.
In view of this, a bearing bush formed of synthetic resin is arranged between the forward end of the small diameter portion of the housing and the spline shaft to thereby reduce the moment load acting on the spline nut. While this bearing bush is forced into the inner peripheral surface of the small diameter portion of the housing with pressure, to suppress an increase in the sliding resistance of the housing with respect to the spline shaft, the bearing bush is fitted onto the spline shaft in a state of a so-called clearance fit through the intermediation of a minute clearance (e.g., approximately 0 to 24 μm when the outer diameter of the spline shaft is 6 mm) with respect to the spline nut.
However, when the resin bearing bush fixed to the small diameter portion of the housing is thus fitted onto the spline shaft in a state of a clearance fit, due to the presence of a clearance, although extremely small, between the spline shaft and the bearing bush, the contact between the bearing bush and the spline shaft becomes rather unstable and the sliding resistance of the housing with respect to the spline shaft undergoes fluctuation or the like, so there arises a problem in that the precision in the axial advancing/retreating movement of the housing is likely to be impaired.
Further, strict control of the inner diameter dimension of the bearing bush and the outer diameter dimension of the spline shaft is required, which leads to an increase in production cost.
As disclosed in JP 2001-12472 A, in a known conventional guide device for a shaft member using a bearing bush formed of synthetic resin, the bearing bush and the shaft member are held in press contact with each other in a state of a so-called interference fit. In this guide device, the interference with which the two members are fit-engaged with each other is controlled by performing dimension control on the inner diameter of the bearing bush and the outer diameter of the shaft member, whereby the sliding resistance of the bearing bush with respect to the shaft member is kept within a predetermined range.